Transmit beamforming is a signal processing technique that uses two or more antennas at a transmitter to increase the signal to interference plus noise ratio (SINR) at a receiver and, thereby, the reliability and/or rate at which data can be communicated from the transmitter to the receiver. The basic operation of transmit beamforming is to send the same signal to the receiver from each of the two or more antennas at the transmitter. However, a different complex scaling factor (or set of complex scaling factors) is applied to the signal sent from each antenna.
In order to improve the SINR at the receiver, the values of the different complex scaling factors (or sets of complex scaling factors) should be determined based on state information (e.g., the frequency or impulse response) of the wireless channel used to convey signals from the transmitter to the receiver. In a time division duplexing (TDD) system, the frequency band of the wireless channel used to convey signals from the transmitter to the receiver is the same frequency band used to convey signals in the opposite direction, from the receiver to the transmitter. Therefore, because the frequency bands are the same, the transmitter can form an estimate of the state information of the wireless channel based on a signal it receives from the receiver and use that estimate to perform transmit beamforming.
In a frequency division duplexing (FDD) system, however, different frequency bands are used to send signals in each direction between the two devices over the wireless channel. As a result, rather than the transmitter forming an estimate of the state information of the wireless channel based on a signal it receives from the receiver and using that estimate to perform transmit beamforming, such a system would conventionally operate in a closed-loop mode to form an estimate of the state information of the wireless channel for use in transmit beamforming. More specifically, in such a system, the receiver would estimate the state information of the wireless channel based on a signal it receives from the transmitter and then feed this estimated state information (or some compressed indication of this estimated state information) back to the transmitter for use in transmit beamforming. Although the closed-loop mode provides a viable solution for estimating the state information of the wireless channel in a FDD system for use in transmit beamforming, such a closed loop mode can be slow to adapt to changing channel conditions (e.g., due to movement of the transmitter and/or receiver) and further increases overhead.
Therefore, what is needed is a method and apparatus that can more quickly and/or more efficiently estimate state information of a wireless channel in a FDD system for transmit beamforming.
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